Preventing corrosion of ferrous metals by ammonia free ammonium nitrate



United States PatentO PREVENTING CORROSION OF FERROUS METALS BY AMMONIA FREE AMMONIUM NITRATE Donald C. Young, Fullerton, Calif assignor, by mesne assignments, to 'Collier Carbon and ChemicalCorporation, a corporation of California No Drawing. Application February 11, 1957 Serial No. 639,223

6 Claims. (Cl. 204-148) This invention relates to corrosion prevention and in particular concerns a novel method for reducing the rate at which stressed areas of ferrous metals are corroded by aqueous solutions of ammonium nitrate.

Within recent years there has been a considerable increase in the use of aqueous ammonium nitrate for fertilizer purposes. Unfortunately, however, aqueous ammonium nitrate solutions are notoriously corrosive with respect to ferrous metals, and .the storing and shipping of such solutions in ordinary iron or steel vessels and applying them to the soil with iron or steel equipment is almost prohibitively expensive because of the rapidity with which ferrous metals corrode. The incorporation of corrosion inhibitors into aqueous ammonium nitrate solutions has been proposed, and to some extent these inhibitors are effective in reducing the rate of corrosion. Insofar as I am aware, however, none of such inhibitors is eifective against the unique type of corrosion which occurs at points or areas of internal stress. It is a well known fact that the rate of corrosion of fabricated ferrous metal is considerably higher at places where internal stresses exist in.

themetal. For example, in welded steel vessels which are used for the storage of corrosive liquids it is very commonly observed that in any given period of time the majority of the corrosion takes place at the welds where internal stresses have been created by the welding process. The same phenomenon is observed with ferrous metal castings in which points or areas of internal stress have been created during the casting operation. By reasons of its occurring substantially only at points or areasof internal stress, thistype of corrosion is commonly referred to as stress corrosion to distinguish it from ordinary chemical or electrolytic corrosion of unstressed surfaces.

I have now found 'that the ammonium nitrate corrosion of ferrous metals at stressed as well as unstressed points and areas may be substantially prevented through the use of a particular chemical corrosion inhibitor in combination with a sacrificial metal anode. More particularly, I have found that in operations wherein aqueous ammonium nitrate is maintained in contact with ferrous metal surfaces, corrosion of the latter can be substantially prevented by adding to the ammonium nitrate solution a relatively minor amount of ammonium phosphate and electrically connecting the ferrous metal to a metal anode which is maintained in contact with the ammonium nitrate solution and which is constructed of lead. The use of such a lead anode in conjunction with ammonium phosphate constitutes a true combination since the lead anode will not function to prevent stress corrosion in the absence of the phosphate, nor will the latter similarly function in the absence of the lead anode.

2,874,105 Patented Feb. 17, 1959 I am aware that the principle of cathodic protection has been applied to the present problem. U. S. Patent No. 2,366,796 discloses a process for preventing the corrosion of ferrous metals by ammoniacal ammonium nitrate solutions wherein a low voltage direct current is applied across an electrical circuit comprising the ferrous metal (in a passivated state) as the anode, the ammonium nitrate solution as the electrolyte, and an inert metal cathode. The method of the present invention differs from such prior art method in three important respects: (1) the electrolyte comprises ammonium phosphate in addition to the ammonium nitrate, (2) the ferrous metal which is to be protected is not passivated and acts as a cathode rather than as an anode, and (3) no electric current is supplied from an outside source.

Considering now the invention in detail, the process is generally applicable to ammonium nitrate solutions which normally induce stress corrosion of ferrous metals. Typically, such solutions contain between about 50v and about 68 percent by weight of ammonium nitrate and between about 50 and about 32 percent by weight of water, but while the process of the invention will find widest application with respect to such solutions, it is not limited thereto. It may be pointed out, however, that it is not applicable with respect to ammonium nitrate solutions which contain appreciable amounts of free ammonia.

' The ammonium phosphate which is added to the ammonium nitrate solution may be either mono-ammonium dihydrogen phosphate or di-ammonium monohydrogen phosphate or a mixture of the two. It may be employed in solid form and directly dissolved in the ammonium nitrate solution, or it may first be dissolved in water to form a relatively concentrated solution which is then added to the aqueous ammonium nitrate. Alternatively, and preferably, the ammonium phosphate is employed in the form of the commercially available aqueous fertilizer composition having a fertilizer designation 8 -24-0, meaning that it contains about 8 percent of nitrogen and about 24, percent of phosphorous calculated as P 0 Such composition is an aqueous solution containing about 50-60 percent by weight of ammonium monohydrogen and dihydrogen phosphates in such proportion that its pH value is about 6.5. Being a liquid which ismiscible with aqueous ammonium ni-' trate, it is of course very readily admixed therewith at ordinary temperaturesin a tank or otherv vessel equipped witha -simple stirring device. When the ammonium phosphate is employed in solid form it may be advisable to provide heating means in the mixing vessel to promote rapid solution in the aqueous ammonium'nitrate.

The amount of ammonium phosphate employed depends somewhat upon the concentration of the aqueous ammonium nitrate, aswell as upon the temperature at which the composition is to be stored and the nature of the ferrous metalwithwhich it comes in contact. In general, however, at least about 0.0005 part by weight of the ammonium phosphate should be provided .per part by weight of ammonium nitrate, and since substantially no improvement in corrosion inhibition is attained above about 0.01 part by weight of the phosphate'per part by weight of the nitrate it is not necessary to exceed this latter proportion. If desired, however, con siderably greater amounts of the phosphate may be employed; theminimum amount is of course an amount sufiicient to decrease substantially the rate of corrosion of ferrous metals by the ammonium nitrate solution. Ordinarily, in treating ammonium nitrate solutions of 50-60 percent concentration, between about monium phosphate and in electrical contact with the.

ferrous metal which is to be protected is lead. Such lead anode may take the form of sheets or rods suspended in the aqueous solution and connected to the.

ferrous metal by a copper or other metallic conductor. Where the solution is contained in a relatively shallow ferrous metal vessel the anode may take the form of pellets or the like lying on the bottom of the vessel. Alternatively, when the solution is stored in a ferrous metal tank or shipped in a tank car, the anode is preferably employed in the form of a bar which is bolted or welded to the tank in direct contact therewith. In order that a large part of the solution be in relatively close proximity to the anode, a number of the latter may be provided spaced more or less uniformly throughout the solution. In a typical storage tank installation, wherein a 58 percent aqueous ammonium nitrate solution is stored in' 500 gallon welded horizontal mild steel tanks approximately 42" in diameter and 44" in length, substantially complete inhibition of both surface corrosion and stress corrosion in the tank has been attained by adding about 0.005 gallon of the aforesaid 8-24-0 solution to each gallon of the ammonium nitrate solution and providing two lead anodes in the form of 2" rods, 3 in length, bolted directly to the tank walls adjacent the opposite ends of the tank. The size or weight of the lead anode will depend upon the amount of ammonium nitrate solution involved and the amount of ferrous metal to be protected, as well as on the temperature to which the ferrous metal is subjected in contact with the ammonium nitrate solution. Also, since the anode is slowly consumed, the size or weight of the anode will depend on the frequency at which it is convenient to replace the same. Sufliceth to say, the anode should of course be'of such size that it eifects a substantial reduction in the rate of corrosion at the stressed areas of the ferrous metal for an appreciable period of time. c. g., at least about 30 days under the particular conditions involved.

In order to demonstrate the effectiveness of the method of the invention, the following accelerated test procedure has been employed: A mild steel wire test specimen,

0.024" in diameter and having a breaking strength of about 46,600 p. s. i., is artificially stressed by being heated to 1300" F. for 3 minutes followed by quenching in water. The stressed wire has a breaking strength of about 28,100 p. s. i. The stressed wire is then mounted in a round-bottom chemical glass flask in such manner that it passes horizontally through the fiask at about its mid-point. One end of the wire is fixed by a clamp outside the flask, and the other end is weighted outside the flask so that a tension of 28,000 p. s. i. is placed on the wire. The flask is filled with the test solution, and a metallic anode is suspended in the solution from the neck of the flask. The anode is electrically connected to the fixed end of the test wire via a copper conductor. Heating means are provided to maintain a test solution at any desired temperature. The rate at which the stressed wire is corroded by the test solution is measured by observing the time required for the wire .to break under the imposed tension. The following data are typical and are illustrative of the operability of the invention:

Test Test solution Temlp Anode Breaking N 0. time 1 58% aqueous ammoniumnltrate 150 None. 3.5 hours.

solution. 2 o .i".-. 150 Lead" Do. 3. 58% aqueous ammonlumnitrate 150 None- 5.0 hours.

solution +05% by weight of 8-24-0." 4 do 150 Lead 2,500 hours. 5 58%aqueous ammonlumnitrate 234 None 12 minutes.

solution. 6 o 234 -.do Do. 7"..- 58% aqueous ammonium nitrate 234 ..-do Do.

s 8lig2lp51;|-0.5% by weight 0t 8..- 58% aqueous ammonlumnltrate 234 do 13 minutes.

solution +13% by weight of 8-24-0." 9 58% aqueous ammonlumnitrate 234 do Do.

59115111051 +10% by weight of 8- 4- 10...- 58% aqueous ammoniumnitrate 234 Lead.- 1,000 hours.

srglutil i +0.5% by weight of i.

1 Test discontinued at end of stated period.

Each tank was 1' in diameter and 2' long and was mounted horizontally on steel legs. The test solution employed was an aqueous ammonium nitrate solution corresponding to the fertilizer designation 20-0-0. The anodes were lead rods, 1 in length. In one of the tanks, a single anode was mounted vertically about midway along the length of the tank. In a second tank, an anode Was mounted vertically adjacent each end of the tank, and in a third tank anodes were mounted adjacent each end and about midway alongthe length. In all instances the anodes were electrically grounded to the walls of the tanks. A fourth tank had no anodes installed. A11 tanks were mounted in an oven which was maintained at about 150 F. Approximately 0.5 percent by weight of aqueous ammonium phosphate (8-24-0) was added to the test solution, and the tanks were filled with the resulting composition. Periodically the tanks were emptied and inspected for evidence of corrosion along the welded seams, i. e., at stressed areas. The following observations were made:

' Tank #1 (1 anode)Corrosion at stressed areas observed after 20 days. No general corrosion.

Tank #2 (2 anodes)-No corrosion at stressed areas ob-- served after 32 days. No general corrosion.

Tank #3 (3 anodes)-No corrosion at stressed areas observed after 37 days. No general corrosion.

Tank #4 (No anode)-Corrosion at stressed areas observed after 4 days. No general corrosion; Other modes of applying the principle of my invention may be employed instead, of those explained, change being made as regards the methods or materials employed,

provided the step or steps stated by any of the following.

claims, or the equivalent of such stated employed.

I, therefore, particularly as my invention: p I

1. In a process wherein an essentially ammonia-free aqueous solution of ammonium nitrate is maintained in contact with a ferrous metal comprising areas of internal, stress subject to corrosion by said solution, the method of reducing the rate of said corrosion which comprises adding ammonium phosphate to said solution and maintaining a body of lead in physical contact with the resulting composition and in electrical contact with said ferrous step or steps, be

point out and distinctly claim metal, said ammonium phosphate being present in an amount corresponding to at least about 0.0005 part by weight per part by weight of the ammonium nitrate.

2. The process of claim 1 wherein said aqueous solution contains between about 50 and about 60 percent by weight of ammonium nitrate.

3. The process of claim 1 wherein the said ammonium phosphate is employed in the form of an aqueous solution containing between about 50 and about 60 percent by weight of ammonium phosphates and having a pH value of about 6.5 and analyzing about 8 percent by weight nitrogen and about 24 percent by weight phosphorus calculated as P 4. The process of claim 1 wherein between about 0.0005 part and about 0.1 part by weight of ammonium phosphate is employed per part by weight of ammonium nitrate.

5. The process of claim 3 wherein the said ammonium nitrate solution contains between about 50 and about 60 percent by weight of ammonium nitrate and between about 0.05 and about 1.0 part by weight of said ammonium phosphate solution is provided for each 100 parts by weight of said ammonium nitrate solution.

6. In a process wherein an essentially ammonia-free aqueous solution of ammonium nitrate of between about 50 and about 60 percent by weight concentration is stored in a ferrous metal vessel comprising areas of internal stress subject to corrosion by said solution, the method of reducing the rate of said corrosion which comprises adding ammonium phosphate to said solution in an amount corresponding between about 0.005 and about 0.1 part per p maintaining a ammonium nitrate, and hysical contact with the solution so obtained and in electrical contact with said vessel, said lead being provided in amount sufficient to 5 effect for an appreciable period of time a substantial reduction in the rate of areas.

said corrosion at References Cited in the file of this patent Evans: Metallic Corrosion, 1948, Edward Arnold and Co.,

UNITED STATES PATENTS Dahmen Feb. 17, Moore Oct. 24, Kircher et al. Apr. 7, Lawrence et al. Oct. 6, Kniskern et al. Ian. 19, Kniskern et al. May 10, Menaul Mar. 7, Beekhuis et al. Sept. 17, Lawrence et al. Jan. 6, Gerhold Feb. 5, Crittenden Apr. 17,

OTHER REFERENCES 283, 535, 536 and 556.

said stressed Passivity and Protection,

London. Pages 14, 

1. IN A PROCESS WHEREIN AN ESSENTIALLY AMMONIA-FREE AQUEOUS SOLUTION F AMMONIUM NITRATE IS MAINTAINED IN CONTACT WITH A FERROUS METAL COMPRISING AREAS OF INTERNAL STREE SUBJECT TO CORROSION BY SAID SOLUTION, THE METHOD OF REDUCING THE RATE OF SAID CORROSION WHICH COMPRISES ADDING AMMONIUM PHOSPHATE TO SAID SOLUTION AND MAINTAINING A BODY OF LEAD IN PHYSICAL CONTACT WITH THE RESULTING COMPOSITION AND IN ELECTRICAL CONTACT WITH SAID FERROUS METAL, SAID AMMONIUM PHOSPHATE BEING PRESENT IN AN AMOUNT CORRESPONDING TO AT LEAST ABOUT 0.0005 PART BY WEIGHT PER PART BY WEIGHT OF THE AMMONIUM NITRATE. 